Expendable liners have been employed to protect the permanent refractory lining of a metal carrying vessel such as a ladle or tundish. These expendable liners are usually fabricated from a composition having heat insulating properties either in the form of premolded boards or gunnable materials. Recently, the technology in this area has shifted from performed boards or gunnable compositions to sprayable insulating compositions that consist essentially of a particulate refractory material premixed with a binder and sprayed in the form of an aqueous mixture onto the permanent refractory surface of a metal carrying vessel like a tundish. The sprayable compositions are designed so that they can be sprayed into either a warm or a cold tundish. The sprayable compositions are based on magnesite refractory grain so that long casting times with corrosive steels and slags are possible. In addition to the magnesite refractory grain, the sprayable insulating mixes contain additives to control the density of the sprayed mixture and binders to produce a strong coating after drying. The sprayable mixture usually contains about 25% water, thus there is no dust during spraying and virtually zero rebound. The sprayable compositions can be sprayed at very high application rates usually on the order of about 6,000 to 9,000 pounds per hour. Typically, about the same weight of material is used to spray a tundish as the weight of tundish boards required to line the same tundish.
One of the essential features of a sprayed tundish coating compared to a gunnable tundish liner is lower thermal conductivity which reduces energy losses from the steel; thus allowing reduced metal temperature in the ladle and basic oxygen furnace. This extends the useful life of the refractories in those vessels. The low thermal conductivity and high thermal expansion properties of the sprayed coating are essential so that the tundish may be easily stripped of these expendable coatings. The low thermal conductivity produces significantly lower temperatures behind the hot face of the sprayed coating and at the interface with the permanent tundish refractory liner. Maintaining low temperatures behind the hot face and at the permanent refractory interface prevents reaction and/or sintering and thus gives a friable material layer behind the hot face that provides easy stripping of the remaining skull after casting. Thus, sprayed liners reduce wear on the permanent tundish refractory liner.
One of the current problems with sprayable insulating liner compositions is their tendency to degrade the permanent refractory liner and cause liquid formation at operating temperatures which in turn causes sticking of the skull. In other instances, even if the skull does not stick to the permanent refractory surface, compositions cause contamination of the permanent refractory surface thus producing spalling due to the increase in thermal expansion of the surface compared to the interior of the refractory. Sticking of the skull to the refractory causes some of the permanent refractory to be pulled out of the metal vessel when the skull is mechanically removed.